UV-crosslinkable copolymers

ABSTRACT

The present invention relates to the use of acetophenone or benzophenone derivatives of the formula I 
                         
where
 
R is unsubstituted or substituted
         C 1  to C 3  alkyl,   C 6  to C 10  aryl or   aralkyl having from 6 to 10 carbon atoms in the aryl moiety and from 1 to 6 carbon atoms in the alkyl moiety
 
as copolymerizable photoinitiators.
       
     The invention further relates to copolymers containing acetophenone or benzophenone derivatives of the formula I in copolymerized form, to processes for preparing said copolymers, and to the use of said copolymers in UV crosslinkable compositions.

The present invention relates to the use of acetophenone or benzophenonederivatives of the formula I (referred to below for short as“copolymerizable photoinitiators”),

where

R is unsubstituted or substituted

-   -   C₁ to C₃ alkyl,    -   C₆ to C₁₀ aryl or    -   aralkyl having from 6 to 10 carbon atoms in the aryl moiety and        from 1 to 6 carbon atoms in the alkyl moiety        as copolymerizable photoinitiators.

The invention further relates to copolymers containing acetophenone orbenzophenone derivatives of the formula I in copolymerized form, toprocesses for preparing said copolymers, and to the use of saidcopolymers in UV crosslinkable compositions.

UV crosslinkable adhesives containing photoinitiators in copolymerizedform are known, for example, from the text DE-A 2411169.

Copolymerizable benzophenone or acetophenone derivatives are described,for example, in the texts EP-A 346788 and EP-A 377199.

The text EP-A 246848 discloses pressure sensitive adhesives for use onskin which comprise among other ingredients a crosslinked copolymer. Thecopolymer is prepared using ethylenically unsaturated aromatic ketonederivatives with a broad variety of substituents.

Copolymerizable photoinitiators are to be easy to prepare and are toundergo copolymerization readily, and the copolymers comprisingphotoinitiators are to exhibit good performance properties in use,particularly high cohesion and adhesion when used as adhesives.Moreover, copolymers comprising the photoinitiators are to be stable tohydrolysis in the course of their preparation, storage, and application.

It is an object of the present invention to provide novelcopolymerizable photoinitiators, copolymers containing thesephotoinitiators in copolymerized form, which, when used as adhesives,exhibit not only improved cohesion and adhesion but also increasedstability to hydrolysis.

We have found that this object is achieved by the copolymerizablephotoinitiators defined at the outset, copolymers containing them, andtheir use in UV crosslinkable compositions.

In the copolymerizable photoinitiators of the formula I used inaccordance with the invention

R denotes unsubstituted or substituted

-   -   C₁ to C₃ alkyl, such as methyl, ethyl, n-propyl or iso-propyl,        preferably methyl,    -   C₆ to C₁₀ aryl, such as phenyl or naphthyl, preferably phenyl,        or    -   aralkyl having from 6 to 10 carbon atoms in the aryl moiety and        from 1 to 6 carbon atoms in the alkyl moiety, an example being        benzyl.

Particularly suitable substituents include halogens, such as fluorine,chlorine, and bromine, C₁ to C₆ alkyl groups, such as methyl, ethyl,n-propyl or iso-propyl, n-butyl, iso-butyl or tert-butyl, n-pentyl,iso-pentyl or tert-pentyl, n-hexyl, iso-hexyl or tert-hexyl, estergroups, such as methoxycarbonyl and ethoxycarbonyl, and alkoxy groups,such as methoxy, ethoxy or n-butoxy. The radicals R may contain up to 1,2 or 3 of these substituents.

It is significant that the vinyloxy group in compounds of the formula Ican be in position 2, 3 or 4 with respect to the carbonyl group.Mixtures of these compounds can of course also be used. Preferably,however, in the copolymerizable photoinitiators used, the vinyloxy groupis in position 2 or 4 and in particular in position 4 with respect tothe carbonyl group.

Examples of copolymerizable photoinitiators used include2-vinyloxyacetophenone, 3-vinyloxyacetophenone, 4-vinyloxyacetophenone,2-vinyloxybenzophenone, 3-vinyloxybenzophenone or4-vinyloxybenzophenone, preferably 2-vinyloxyacetophenone,4-vinyloxyacetophenone, 2-vinyloxybenzophenone or 4-vinyloxybenzophenoneand especially 4-vinyloxyacetophenone or 4-vinyloxybenzophenone, aloneor in a mixture.

The preparation of the copolymerizable photoinitiators is known to theskilled worker and takes place, for example, by a metal salt catalyzedreaction of the corresponding phenol compounds II where R is as definedabove with acetylene:

The copolymers of the invention are obtained by free-radicalpolymerization of a mixture of ethylenically unsaturated compounds(monomers) containing the copolymerizable photoinitiators normally in atotal amount of from 0.01 to 10% by weight, preferably from 0.05 to 5%by weight and with particular preference from 0.1 to 2% by weight, basedin each case on the total monomer amount. Accordingly, thecopolymerizable photoinitiators are copolymerized in the copolymers inamounts of from 0.01 to 10% by weight, preferably from 0.05 to 5% byweight, and with particular preference from 0.1 to 2% by weight. At thispoint it may be stated that the percentage amounts specified in thedescription for the ethylenically unsaturated copolymerizablephotoinitiators copolymerized in the copolymer, and for the othermonomers, are generally intended to correspond to the amounts of thesecomponents in the monomer mixture to be polymerized, and vice versa.

In addition to the abovementioned photoinitiators, the copolymersinclude generally predominant fractions, usually from 50 to 99.99% byweight, preferably from 70 to 97.5% by weight, of principal monomerscomprising esters of preferably C₃ to C₆ α, β-monoethylenicallyunsaturated monocarboxylic and dicarboxylic acids, such as particularlyacrylic acid, methacrylic acid, maleic acid, fumaric acid, and itaconicacid, with alkanols having generally from 1 to 12, preferably from 1 to8, and in particular from 1 to 4, carbon atoms, such as particularlymethyl, ethyl, n-butyl, iso-butyl and 2-ethylhexyl acrylate andmethacrylate, dimethyl maleate or di-n-butyl maleate, in copolymerizedform. Suitable comonomers include in particular monomers which are easyto polymerize free-radically, such as ethylene, vinylaromatic monomers,such as styrene, α-methylstyrene, o-chlorostyrene or vinyltoluenes,esters of vinyl alcohol and C₁ to C₁₈ monocarboxylic acids, such asvinyl acetate, vinyl propionate, and vinyl n-butyrate, vinyl laurate,and vinyl stearate, nitriles of α,β-monoethylenically unsaturatedcarboxylic acids, such as acrylonitrile, and also C₄₋₈ conjugateddienes, such as 1,3-butadiene and isoprene, for example.

Additionally to the abovementioned monomers, the copolymers contain withparticular advantage from 0.1 to 15% by weight, preferably from 0.5 to8% by weight, of C₃ to C₆ α,β-monoethylenically unsaturatedmonocarboxylic and dicarboxylic acids, their amides and/or theiranhydrides, such as particularly acrylic acid, methacrylic acid, maleicacid, fumaric acid, and itaconic acid, acrylamide and methacrylamide,maleic anhydride, and also vinylsulfonic acid,2-acrylamido-2-methylpropanesulfonic acid, styrenesulfonic acid, and thewater-soluble salts thereof, and also N-vinylpyrrolidone andN-vinylimidazole.

Besides the monomers already mentioned, the copolymers may furthercontain other comonomers in copolymerized form, examples being thosewhich commonly increase the internal strength of the films formed fromthe copolymers. These ethylenically unsaturated monomers normallycontain at least one epoxy, hydroxyl, N-methylol or carbonyl group, orat least two nonconjugated ethylenically unsaturated double bonds.Examples thereof are N-alkylol amides of C₃ to C₁₀ α,β-monoethylenically unsaturated carboxylic acids, among which veryparticular preference is given to N-methylolacrylamide andN-methylolmethacrylamide, and also their esters with alkanols havingfrom 1 to 4 carbon atoms. Also suitable, moreover, are monomerscontaining two vinyl radicals, monomers containing two vinylideneradicals, and monomers containing two alkenyl radicals. Particularlyadvantageous here are the diesters of dihydric alcohols withα,β-monoethylenically unsaturated monocarboxylic acids, among whichpreference is given to acrylic acid and methacrylic acid. Examples ofsuch monomers containing two nonconjugated ethylenically unsaturateddouble bonds are alkylene glycol diacrylates and dimethacrylates such asethylene glycol diacrylate, 1,2-propylene glycol diacrylate,1,3-propylene glycol diacrylate, 1,3-butylene glycol diacrylate,1,4-butylene glycol diacrylates, and ethylene glycol dimethacrylate,1,2-propylene glycol dimethacrylate, 1,3-propylene glycoldimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butylene glycoldimethacrylates, and also divinylbenzene, vinyl methacrylate, vinylacrylate, allyl methacrylate, allyl acrylate, diallyl maleate, diallylfumarate, methylenebisacrylamide, cyclopentadienyl acrylate, triallylcyanurate or triallyl isocyanurate. Also of particular importance inthis context are the C₁–C₈ hydroxyalkyl esters of acrylic andmethacrylic acid, such as n-hydroxyethyl, n-hydroxypropyl orn-hydroxybutyl acrylate and methacrylate, and also compounds such asdiacetone acrylamide and acetylacetoxyethyl acrylate and methacrylate.In accordance with the invention, the aforementioned monomers arefrequently copolymerized in amounts of from 0.1 to 10% by weight, basedon the total amount of the monomers to be polymerized.

The manner in which the monomeric components are added to thepolymerization vessel in the course of the free-radical polymerizationis known to the skilled worker. They may either be included in theirentirety in the initial charge to the polymerization vessel or elseadded continuously or in stages at the rate at which they are consumedin the course of the free-radical polymerization. In each specific casethis will depend on the chemical nature of the initiator system and onthe polymerization temperature. Preferably, a small portion of themonomeric components is included in the initial charge and the remainderis supplied to the polymerization zone at the rate at which it isconsumed. It is of course also possible to alter the composition of themonomer mixture to be polymerized in the course of the polymerization.These process variants are known to the skilled worker. Thus in what isknown as the staged procedure, for example, a monomer mixture 1 issupplied first and then a monomer mixture 2 having a different monomercomposition is supplied subsequently to the polymerization vessel at therate at which they are consumed, with the composition of the monomermixture supplied to the polymerization vessel being modifiedcontinuously in what is known as a gradient procedure. Thepolymerization frequently takes place under an inert gas atmosphere,under nitrogen or argon, for example.

The copolymers of the invention normally have K values of from 10 to 150and often from 15 to 100. The K values are normally determined at 25° C.in accordance with DIN ISO 1628-1 using a 1% strength by weight solutionof the copolymers in tetrahydrofuran. The K value is preferably from 25to 55 if the copolymers are to be used as hotmelt pressure sensitiveadhesives. Where the copolymers are to be used in UV-curablecompositions for coating mineral surfaces, their K values are preferablyfrom 60 to 100. Copolymers which are to be used for coating materialspreferably have K values of from 15 to 85.

The copolymers of the invention may have glass transition temperaturesof from −70 to +150° C. Depending on the intended use, there isfrequently a requirement for copolymers whose glass transitiontemperatures are situated within particular ranges. By means ofappropriate selection of the ethylenically unsaturated monomers to bepolymerized it is possible for the skilled worker deliberately toprepare copolymers whose glass transition temperatures are situatedwithin the desired range. Where, for example, the copolymers of theinvention are to be used as pressure sensitive adhesives, thecomposition of the monomer mixture to be polymerized is selected suchthat the copolymers produced have glass transition temperatures of <0°C., frequently ≦−5° C., and often ≦−10° C. Where the copolymers are tofind use, however, as UV-curable binders in coating formulations, thecomposition of the monomer mixture to be polymerized is selected suchthat the copolymers produced have glass transition temperatures of from−40 to +150° C., frequently from 0 to +100° C., and often from +20 to+80° C.

The glass transition temperature, T_(g), is the limiting value of theglass transition temperature toward which it tends with increasingmolecular weight according to G. Kanig (Kolloid-Zeitschrift &Zeitschrift für Polymere, Vol. 190, page 1, equation 1). The glasstransition temperature is determined by the DSC method (DifferentialScanning Calorimetry, 20 K/min, midpoint measurement, DIN 53 765).

According to Fox (T. G. Fox, Bull. Am. Phys. Soc. 1956 [Ser. II] 1, page123 and in accordance with Ullmann's Encyclopädie der technischenChemie, Vol. 19, page 18, 4th edition, Verlag Chemie, Weinheim, 1980),the glass transition temperature of copolymers with no more than aslight degree of crosslinking is given in good approximation by:1/T _(g) =x ¹ /T _(g) ¹ +x ² /T _(g) ² + . . . x ^(n) /T _(g) ^(n),where x¹, x², . . . x^(n) are the mass fractions of the monomers 1, 2, .. . n and T_(g) ¹, T_(g) ², . . . T_(g) ^(n) are the glass transitiontemperatures of the polymers composed in each case of only one of themonomers 1, 2, . . . n, in degrees Kelvin. T_(g) values for thehomopolymers of the majority of the monomers are known and are listed,for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed.,Vol. A21, page 169, VCH Weinheim, 1992; other sources of glasstransition temperatures of homopolymers are constituted, for example, byJ. Brandrup, E. H. Immergut, Polymer Handbook, 1st Ed., J. Wiley, NewYork 1966, 2nd Ed. J. Wiley, New York 1975, and 3rd Ed. J. Wiley, NewYork 1989.

The novel copolymers can be prepared by copolymerizing the monomericcomponents using the customary polymerization initiators and also, whereappropriate, regulators, polymerization being carried out at thestandard temperatures without solvent, in emulsion, in water or insuitable organic media, for example, or in solution. The novelcopolymers are preferably prepared by polymerizing the monomericcomponents in organic solvents, especially in solvents having a boilingrange of from 50 to 150° C., preferably from 60 to 120° C., using thecustomary amount of polymerization initiators, which is generally from0.01 to 10% by weight, in particular from 0.1 to 4% by weight, based ineach case on the total weight of the monomeric components. Particularlysuitable organic solvents include alcohols, such as methanol, ethanol,n- and iso-propanol, n- and iso-butanol, cyclic ethers, such astetrahydrofuran, and hydrocarbons, such as toluene and petroleum spiritswith a boiling range of from 60 to 120° C. It is also possible to useketones, such as acetone, methyl ethyl ketone, and methyl isobutylketone, and esters, such as ethyl acetate, and also mixtures of solventsof said type, preference being given to mixtures containing iso-butanoland/or methyl ethyl ketone in amounts of ≧70% by weight, especially ≧80%by weight, and in particular ≧90% by weight, based on the solventmixture employed.

The manner in which the solvent or solvent mixture is added to thepolymerization vessel in the course of the free-radical polymerizationis known to the skilled worker. It may either be included completely inthe initial charge to the polymerization vessel or else insertedcontinuously or in stages in the course of the free-radicalpolymerization. The solvent may also be used in a mixture with themonomers and/or the initiator. It is preferred to include a majorfraction of the solvent in the initial charge and to supply theremainder to the polymerization zone together with the monomers to bepolymerized and/or initiators.

Examples of suitable polymerization initiators for solutionpolymerization include azo compounds, such as2,2′-azobisiso-butyronitrile, 2,2′-azobis-2-methylbutyronitrile, diacylperoxides, such as dibenzoyl peroxide, dilauroyl peroxide, didecanoylperoxide, and diisononanoyl peroxide, alkyl peresters, such astert-butyl perpivalate, tert-butyl per-2-ethylhexanoate, tert-butylpermaleate, tert-butyl perisononanoate, tert-butyl perneodecanoate, andtert-butyl perbenzoate, dialkyl peroxides, such as dicumyl peroxide ordi-tert-butyl peroxide, peroxydicarbonates, such as dimyristylperoxydicarbonate, dicetyl peroxydicarbonate,bis(4-tert-butylcyclohexyl) peroxydicarbonate, dicyclohexylperoxydicarbonate, and bis(2-ethylhexyl) peroxydicarbonate,hydroperoxides, such as tert-butyl hydroperoxide, cumene hydroperoxide,alone or in a mixture. In the case of aqueous emulsion polymerization itis possible to use the customary initiators, such as sodium, potassium,and ammonium peroxodisulfate, for example, or else redox systems knownto the skilled worker.

The manner in which the initiator is added to the polymerization vesselin the course of free-radical polymerization is known to the skilledworker. It may either be included completely in the initial charge tothe polymerization vessel or else inserted continuously or in stages atthe rate at which it is consumed in the course of the free-radicalpolymerization. Specifically, this will depend on the chemical nature ofthe initiator system and on the polymerization temperature. Preferably,a small portion is included in the initial charge and the remainder issupplied to the polymerization zone at the rate at which it is consumed.It is frequently favorable to conduct the polymerization reaction suchthat first of all ≦50% by weight, often ≦45% by weight or ≦40% byweight, of the initiator are supplied to the polymerization vesselcontinuously over a relatively long period of time, after which >50%,often ≧55% by weight or ≧60% by weight, of the initiator are supplied tothe polymerization vessel continuously over a shorter period of time.

The polymerization may be conducted conventionally in a polymerizationapparatus, which is generally equipped with a stirrer, two or more feedvessels and/or feed lines, reflux condensers, and heating and coolingdevices, and which is equipped for operating under an inert gasatmosphere and under pressures above or below atmospheric pressure.

Following polymerization in solution, the solvents may be separated off,where appropriate under reduced pressure, operating at elevatedtemperatures of up to 150° C. The novel copolymers may then be used inthe low-solvent or solvent-free state, i.e., in the form of melts, asadhesives, preferably pressure sensitive adhesives and especiallyhotmelt pressure sensitive adhesives, or as UV-curable binders incoating formulations, such as coating materials for coating mineralsurfaces, for example, or as paints. In some cases it may also be ofadvantage to prepare the novel copolymers by copolymerization in bulk,i.e., without the presence of a solvent, in which case it is possible tooperate batchwise or else continuously, in accordance for example withthe teaching of U.S. Pat. No. 4,042,768.

Where the novel copolymers are used in the form of solutions—forexample, as UV curable binders in coating formulations, such as coatingmaterials for coating mineral surfaces or as paints, for example—thecopolymer solutions contain normally from 1 to 900 parts by weight,preferably from 10 to 100 parts by weight, and with particularpreference from 20 to 40 parts by weight, of solvent per 100 parts byweight of copolymer. For these purposes it is frequently possible to usethe copolymer solutions obtainable from the solution polymerizationdirectly, or to prepare formulations from them in a simple way bydilution or concentration.

In some cases, as for example when the novel copolymers are prepared byfree-radically initiated aqueous emulsion polymerization, it is alsopossible to use customary regulators in the customary amounts, such asin amounts of from 0.1 to 10 parts by weight or from 0.5 to 5 parts byweight per 100 parts by weight of the monomers used for thepolymerization. Such regulators are used in order to regulate themolecular weight of the copolymers and are known to the skilled worker.It is common to use mercapto compounds as regulators, such as2-mercaptoethanol, 3-mercaptopropionic acid methyl ester,3-mercaptopropyltrimethoxysilane,3-mercaptopropylmethyldi-methoxysilane, 3-mercaptopropionic acid,n-decyl or tert-decyl mercaptan, 1,6-dimercaptohexane,1,9-dimercaptononane, hydrocarbons, such as cumene, alcohols, such asiso-propanol and iso-butanol, or halogenated hydrocarbons, such ascarbon tetrachloride, carbon tetrabromide, chloroform or bromoform, orethers, such as tetrahydrofuran and dioxane.

Where the novel copolymers are prepared by free-radically initiatedaqueous emulsion polymerization, this polymerization normally takesplace in the presence of dispersants. As dispersants it is possible touse emulsifiers and/or protective colloids which are familiar to theskilled worker, examples being nonionic and also anionic or cationicemulsifiers. Preference is given to using nonionic and anionicemulsifiers. The amount of dispersant is usually up to 30 parts byweight, preferably from 0.5 to 10 parts by weight and with particularpreference from 1 to 6 parts by weight, based on 100 parts by weight ofthe monomers to be polymerized.

After the end of the copolymerization it is common to remove unreactedmonomers from the reaction mixture. In the case of solutionpolymerization this takes place simultaneously with the removal of thesolvent under reduced pressure. In order to increase the efficiencyhere, especially on the plant scale, the copolymer is stripped withsteam at the end of the solvent separation process. This steam strippingfrequently takes place at the end of free-radically initiated emulsionpolymerization likewise, where appropriate following an interveningpostpolymerization step, such as is known to the skilled worker from,for example, the texts WO 95/33775, EP-A 767180, and DE-A 19743759. Itis significant that the photoinitiators of the invention incopolymerized form, as compared with the copolymerizable photoinitiatorsknown from the prior art, possess improved stability toward hydrolysisby water or other protic organic solvents, such as iso-propanol oriso-butanol, for example.

For the use of the novel copolymers, they may be conventionally modifiedand/or processed and used, for example, as hotmelt pressure sensitiveadhesives. Thus it is possible to add, for example, customary tackifyingresins, examples being hydrocarbon resins, modified natural rosins orchemically modified rosins, which are composed predominantly of abieticacid or abietic acid derivatives, cumarone-indene resins,terpene-phenolic resins, aldehyde resins or homopolymers, such aspoly-2-ethylhexyl acrylate or poly-α-methylstyrene, and alsoplasticizers, based for example on monoester, diester or polyestercompounds, perchlorinated hydrocarbons or liquid paraffins, dyes andpigments, or stabilizers or elastomeric substances, such as natural orsynthetic rubber, polyvinyl ethers, and also, furthermore, polybutadieneoils, in amounts of from 0.1 to 50% by weight, based on the total mass.

Further suitable modifying compounds include mono- or polyolefinicallyunsaturated compounds of relatively high molecular mass, such as, forexample, polyesterols and polyetherols esterified with acrylic acid,such as the acrylates of tripropylene glycol, tetraethylene glycol orother polyethylene glycols. Also suitable are diacrylates anddimethacrylates of polytetrahydrofuran with molecular weights ofgenerally from 250 to 2000 (numerical averages). Compounds of this kindcontaining at least diolefinic unsaturation can be used with advantagein amounts of from 0.1 to 10 parts by weight per 100 parts by weight ofcopolymer, particular interest attaching to diolefinically unsaturatedcompounds of this kind having a molecular weight of at least 500(numerical average).

The novel copolymers are particularly suitable as melts, as solutions orin the form of aqueous dispersions for producing coatings, coverings,and impregnated systems, especially pressure sensitive adhesives,pressure sensitive adhesive sheets, and pressure sensitive adhesivelabels, and also hot-stamping foils. The compositions may be appliedconventionally by spreading, spraying, rolling, knifecoating orflowcoating, where appropriate at elevated temperature (usually in thetemperature range from 20 to 150° C.), to customary substrates, examplesbeing paper, card, wood, metals, such as aluminum, polymer films, suchas plasticized PVC, polyethylene, polyamides, polyethylene glycolterephthalate, and polypropylene.

Where solvents are used, they are easy to remove from the coatings byevaporation, where appropriate at room temperature or slightly elevatedtemperatures, generally at temperatures from 20 to 150° C. andpreferably at from 50 to 80° C., in which case use is commonly made ofradiant heaters or hot air circulation devices. The applied films, whereappropriate after (initial) drying, can subsequently be crosslinked byexposure to UV light, giving readily adhering coverings which exhibithigh cohesion in conjunction with good adhesion and excellent agingstability. Irradiation with UV light does not normally require inert gasconditions and can usually take place in air. Suitable UV sourcesinclude the customary sources, examples being low, medium or highpressure mercury vapor lamps, which may have outputs of from 20 to 100J/s×cm². Lamps with a higher output generally allow more rapidcrosslinking. In some cases, during the crosslinking irradiation,residual solvent or water can be removed at the same time by the IRcomponent of the lamps.

The adhesive properties of flat substrates with a film of pressuresensitive adhesive can be ascertained by measuring the shear strength,as a measure of the cohesion, and the peel strength as a measure of thesurface tackiness.

EXAMPLES

I Preparation of the Copolymerizable Photoinitiators

I a) 4-Vinyloxybenzophenone (Photoinitiator A)

A mixture of 650 g of 4-hydroxybenzophenone (98% by weight, Avocado,Research Chemicals Ltd.), 750 ml of N-methylpyrrolidone (>99% by weight,BASF AG) and 60 g of zinc naphthenate (Nusa; zinc salts of naphthenicacids, zinc content 12% by weight) was charged with stirring to a 2.5 lautoclave and flushed with nitrogen. After it had been heated to 190°C., injection was carried out first with nitrogen to 2 bar and then withacetylene to 20 bar. During the reaction, further acetylene was suppliedto replace that consumed by reaction, so as to maintain a constantpressure of 20 bar. After a reaction time of 12 hours the batch wascooled to 20–25° C. (room temperature) and let down to atmosphericpressure. The solvent was distilled off under reduced pressure at abridge temperature of from 70 to 80° C. with a pressure of 1 mbar(absolute). 310 g of Pluriol® E 600 (polyethylene glycol, trademark ofBASF AG) were then added and the product was purified by a Sambaydistillation at 1.2 mbar (absolute) and a bridge temperature of 163° C.This gave a total of 522 g of 4-vinyloxybenzophenone having a purity offrom 96 to 99% by weight and a melting point of 58° C.

I b) 4-Acryloyloxybenzophenone (Comparison Photoinitiator)

4-Acryloyloxybenzophenone was prepared in accordance with EP-A 246848,Example A.

II Preparation of the Copolymers

Example 1

A heatable and coolable 2 l glass reactor equipped with an anchorstirrer, reflux condenser, evacuating means and metering means wascharged at room temperature and under nitrogen with

-   108.5 g of iso-butanol (IB; 99.5% by weight)    -   50.5 g of feed stream 1 and        -   4.3 g of feed stream 2            and the closed apparatus was heated with stirring to 100° C.            without pressure compensation. Beginning simultaneously and            at this temperature, the remainder of feed stream 1 was            metered in over the course of 3 hours and the remainder of            feed stream 2 over the course of 3.5 hours. 15 minutes after            the end of feed stream 2, feed stream 3 was commenced, and            was metered in over the course of 15 minutes. Simultaneously            with the metering of feed stream 3, the temperature was            raised to 120° C.

After the end of feed stream 3, postpolymerization was conducted at 120°C. for one hour more. The temperature was then lowered to 100° C., theoverpressure was carefully let down to atmospheric pressure, andthereafter the solvent and the other low boilers were distilled off bycareful application of reduced pressure down to a final pressure of 10mbar (absolute). The reaction batch was then cooled to room temperature.

Feed Stream 1

-   491.0 g of n-butyl acrylate (nBA; ≧99.5% by weight, BASF AG)-   278.5 g of 2-ethylhexyl acrylate (EHA; ≧99.6% by weight, BASF AG)-   189.0 g of methyl methacrylate (MMA; ≧99.9% by weight, BASF AG)    -   23.0 g of acrylic acid (AA; ≧99.0% by weight, BASF AG)        -   4.3 g of photoinitiator A            Feed Stream 2    -   41.7 g of IB        -   0.3 g of tert-butyl per-2-ethylhexanoate (TBEH; ≧98.5% by            weight, Peroxid-Chemie GmbH)            Feed Stream 3    -   16.7 g of IB        -   2.0 g of TBEH

A clear polymer of high viscosity was obtained which had a solidscontent of >99.9% by weight.

The solids content was generally determined by heating 1–2 g of theresulting polymer in an aluminum crucible with a diameter of about 3 cmat 140° C. under atmospheric pressure to constant weight. Twomeasurements were conducted in each case. The figures stated representthe averages from these measurements. In all of the following examples,solids contents >99.9% by weight were likewise found.

The K value of the copolymer was 49.9.

The K values of the copolymers were generally determined in accordancewith H. Fikentscher, Cellulosechemie 1932 (13) pages 58 to 64 and pages71 to 74, with K=k×10³. The measurements were conducted at 25° C. on a1% strength by weight solution of the copolymers in THF (in accordancewith DIN ISO 1628-1).

Comparative Example 1

Comparative example 1 was like Example 1 except that instead ofphotoinitiator A the same amount of the comparison photoinitiator wasused. The K value measured was 50.5.

Example 2

Example 2 was like Example 1 except that instead of IB methyl ethylketone (MEK; ≧99.0% by weight, Deutsche Shell Chemie GmbH) was used. A Kvalue of 50.8 was measured.

Comparative Example 2

Comparative example 2 was like Example 2 except that instead ofphotoinitiator A the same amount of the comparison photoinitiator wasused. The K value measured was 50.2.

Example 3

Example 3 was like Example 1 except that the TBEH in feed streams 2 and3 was replaced by 2,2′-azobis-2-methylbutyronitrile (Wako V59, WAKOChemicals GmbH). The K value measured was 51.2.

Example 4

A heatable and coolable 2 l glass reactor equipped with an anchorstirrer, reflux condenser, evacuating means and metering means wascharged at room temperature and under nitrogen with

-   110.5 g of IB    -   52.5 g of feed stream 1 and        -   4.0 g of feed stream 2            and the closed apparatus was heated with stirring to 100° C.            without pressure compensation. Beginning simultaneously and            at this temperature, the remainder of feed stream 1 was            metered in over the course of 3 hours and the remainder of            feed stream 2 over the course of 3.5 hours. 15 minutes after            the end of feed stream 2, feed stream 3 was commenced, and            was metered in over the course of 15 minutes. Simultaneously            with the metering of feed stream 3, the temperature was            raised to 120° C.

After the end of feed stream 3, postpolymerization was conducted at 120°C. for one hour more. The temperature was then lowered to 100° C., theoverpressure was carefully let down to atmospheric pressure, andthereafter the solvent and the other low boilers were distilled off bycareful application of reduced pressure down to a final pressure of 10mbar (absolute). The reaction batch was then cooled to room temperature.A clear polymer of high viscosity was obtained having a solids contentof >99.9% by weight. The K value was found to be 48.8.

Feed Stream 1

-   422.0 g of nBA-   347.5 g of EHA-   189.0 g of MMA    -   25.0 g of AA    -   4.3 g of photoinitiator A        Feed Stream 2    -   41.7 g of IB        -   0.4 g of TBEH            Feed Stream 3    -   16.7 g of IB        -   2.4 g of TBEH

Example 5

Example 5 was like Example 4 except that MEK was used instead of IB. A Kvalue of 50.7 was measured.

Example 6

Example 6 was like Example 4 except that 5.0 g of photoinitiator A wereused. The K value measured was 48.3.

Example 7

Example 7 was like Example 4 except that 3.6 g of photoinitiator A wereused. The K value measured was 48.3.

Example 8

A heatable and coolable 2 l glass reactor equipped with an anchorstirrer, reflux condenser, evacuating means and metering means wascharged at room temperature and under nitrogen with

-   115.0 g of IB    -   59.5 g of feed stream 1 and        -   3.3 g of feed stream 2            and the closed apparatus was heated with stirring to 100° C.            without pressure compensation. Beginning simultaneously and            at this temperature, the remainder of feed stream 1 was            metered in over the course of 3.5 hours and the remainder of            feed stream 2 over the course of 4 hours. 15 minutes after            the end of feed stream 2, feed stream 3 was commenced, and            was metered in over the course of 15 minutes. Simultaneously            with the metering of feed stream 3, the temperature was            raised to 115° C.

After the end of feed stream 3, postpolymerization was conducted at 115°C. for two hours more. The temperature was then lowered to 100° C., theoverpressure was carefully let down to atmospheric pressure, andthereafter the solvent and the other low boilers were distilled off bycareful application of reduced pressure down to a final pressure of 10mbar (absolute). The reaction batch was then cooled to room temperature.A clear polymer of high viscosity was obtained having a solids contentof >99.9% by weight. The K value was found to be 49.7.

Feed Stream 1

-   1117.0 g of nBA    -   59.1 g of AA        -   4.3 g of photoinitiator A            Feed Stream 2-   65.8 g of IB    -   1.3 g of TBEH        Feed Stream 3-   19.7 g of IB    -   2.6 g of TBEH

Example 9

A heatable and coolable 2 l glass reactor equipped with an anchorstirrer, reflux condenser, evacuating means and metering means wascharged at room temperature and under nitrogen with

-   115.0 g of IB    -   59.5 g of feed stream 1 and        -   3.3 g of feed stream 2            and the closed apparatus was heated with stirring to 100° C.            without pressure compensation. Beginning simultaneously and            at this temperature, the remainder of feed stream 1 was            metered in over the course of 3.5 hours and the remainder of            feed stream 2 over the course of 4 hours. 15 minutes after            the end of feed stream 2, feed stream 3 was commenced, and            was metered in over the course of 15 minutes. Simultaneously            with the metering of feed stream 3, the temperature was            raised to 115° C.

After the end of feed stream 3, postpolymerization was conducted at 115°C. for two hours more. The temperature was then lowered to 100° C., theoverpressure was carefully let down to atmospheric pressure, andthereafter the solvent and the other low boilers were distilled off bycareful application of reduced pressure down to a final pressure of 10mbar (absolute). The reaction batch was then cooled to room temperature.A clear polymer of high viscosity was obtained having a solids contentof >99.9% by weight. The K value was found to be 51.9.

Feed Stream 1

-   1100.0 g of nBA    -   55.1 g of AA    -   22.0 g of maleic anhydride (MAA; >99.7% by weight, Lonza S.P.A.)    -   4.3 g of photoinitiator A        Feed Stream 2-   65.8 g of IB    -   1.3 g of TBEH        Feed Stream 3-   19.7 g of IB    -   2.6 g of TBEH

Example 10

A heatable and coolable 2 l glass reactor equipped with an anchorstirrer, reflux condenser, evacuating means and metering means wascharged at room temperature and under nitrogen with

-   115.0 g of IB    -   59.5 g of feed stream 1 and        -   3.3 g of feed stream 2            and the closed apparatus was heated with stirring to 100° C.            without pressure compensation. Beginning simultaneously and            at this temperature, the remainder of feed stream 1 was            metered in over the course of 3.5 hours and the remainder of            feed stream 2 over the course of 4 hours. 15 minutes after            the end of feed stream 2, feed stream 3 was commenced, and            was metered in over the course of 15 minutes. Simultaneously            with the metering of feed stream 3, the temperature was            raised to 115° C.

After the end of feed stream 3, postpolymerization was conducted at 115°C. for two hours more. The temperature was then lowered to 100° C., theoverpressure was carefully let down to atmospheric pressure, andthereafter the solvent and the other low boilers were distilled off bycareful application of reduced pressure down to a final pressure of 10mbar (absolute). The reaction batch was then cooled to room temperature.A clear polymer of high viscosity was obtained having a solids contentof >99.9% by weight. The K value was found to be 50.1.

Feed Stream 1

-   1043.0 g of nBA    -   133.1 g of AA        -   10.0 g of photoinitiator A            Feed Stream 2-   65.0 g of IB    -   1.7 g of TBEH        Feed Stream 3-   19.9 g of IB    -   2.6 g of TBEH

Example 11

Example 11 was prepared as for Example 10 but using the following feedstreams:

Feed Stream 1

-   1000.0 g of nBA    -   105.1 g of AA        -   72.0 g of MAA        -   5.8 g of photoinitiator A            Feed Stream 2-   68.4 g of IB    -   1.7 g of TBEH        Feed Stream 3-   19.7 g of IB    -   2.8 g of TBEH

A K value of 50.2 was found.

Example 12

A heatable and coolable 2 l glass reactor equipped with an anchorstirrer, reflux condenser, evacuating means and metering means wascharged at room temperature and under nitrogen with

-   110.0 g of IB    -   59.0 g of feed stream 1 and        -   2.5 g of feed stream 2            and the closed apparatus was heated with stirring to 100° C.            without pressure compensation. Beginning simultaneously and            at this temperature, the remainder of feed stream 1 was            metered in over the course of 4 hours and the remainder of            feed stream 2 over the course of 4.5 hours. 15 minutes after            the end of feed stream 2, feed stream 3 was commenced, and            was metered in over the course of 15 minutes. Simultaneously            with the metering of feed stream 3, the temperature was            raised to 115° C.

After the end of feed stream 3, postpolymerization was conducted at 115°C. for two hours more. The temperature was then lowered to 100° C., theoverpressure was carefully let down to atmospheric pressure, andthereafter the solvent and the other low boilers were distilled off bycareful application of reduced pressure down to a final pressure of 10mbar (absolute). The reaction batch was then cooled to room temperature.A clear polymer of high viscosity was obtained having a solids contentof >99.9% by weight. The K value was found to be 49.0.

Feed Stream 1

-   910.0 g of EHA    -   91.0 g of 2-hydroxyethyl acrylate (≧98.5% by weight, BASF AG)    -   10.5 g of photoinitiator A        Feed Stream 2-   35.8 g of IB    -   0.8 g of TBEH        Feed Stream 3-   21.7 g of IB    -   2.8 g of TBEH        III Performance Tests

The copolymers prepared in Examples 1 to 12 were investigated for theirPSA performance properties. The procedure adopted for this was asfollows:

-   a) Preparation of the test strips

The test copolymer was investigated without the addition of tackifiers.On a heatable coating bench at from 85 to 120° C., the copolymer wasapplied in a thin film, using a doctor blade, to a standard commercialpolyester film (Hostaphan film RN 36) and then cooled to roomtemperature. The slot size of the doctor blade was chosen so as to givea copolymer application rate of from 19 to 21 g/m². Irradiation wascarried out with a CK lamp from IST-Strahlentechnik Metz GmbH, which hadan output of 75 mJ/s×cm². For this purpose the coated film was placed ona moving endless belt so that the coated film passed beneath the lamp ata distance of 10 cm and at a speed of 58 m/min. Irradiation was carriedout under air. The sheets produced in this way were cut into strips 2.5cm wide and 25 cm long.

-   b) Testing of the shear strength (in accordance with FINAT FTM 7)

Each test strip was bonded to the edge of a stainless steel test panelso as to give a bond area of 12.5×12.5 mm². 10 minutes after bonding, a1000 g weight was fastened to the protruding end of the sheet and thetest panel was suspended vertically in a space having a temperature of23° C. (constant) and a relative humidity of 50%. The time until thebond breaks under the influence of the weight is a measure of the shearstrength, which in turn represents a measure of the cohesion. The longerthe period of time until the bond breaks, the greater the cohesion. Foreach polymer, three independent measurements were carried out. Thefigures given in Table 1 represent averages from these threemeasurements.

-   c) Testing of the peel strength (in accordance with FINAT FTM 1)

A test strip was bonded to a stainless steel test panel at 23° C. and50% relative humidity.

Following a prescribed contact period of 24 hours, the strip was peeledfrom the test panel at an angle of 180° and a speed of 300 mm per minuteusing a tensile tester machine. The force required to achieve this is ameasure of the adhesion. It is termed the peel strength and is expressedin newtons per 2.5 cm (N/2.5 cm). The higher the value for the peelstrength after the stated time, the higher the adhesion. For eachpolymer, three independent measurements were carried out. The figuresgiven in Table 1 represent averages from these three measurements.

TABLE 1 Overview of the shear strength and peel strength of thecopolymers obtained from Examples 1 to 12 Peel strength in Polymer fromShear strength in N/2.5 cm after 24 Example minutes hours 1 125 12.2Comparative 1 109 9.7 2 122 11.8 Comparative 2 107 9.3 3 130 10.9 4 15011.9 5 151 12.0 6 146 12.3 7 156 12.4 8 124 13.1 9 116 12.2 10  130 10.811  138 9.9 12  123 10.9

As is clearly evident from Table 1, the hotmelt pressure sensitiveadhesives of the invention have much higher shear strengths (cohesion)than a pressure sensitive adhesive prepared using a noninventivephotoinitiator. Likewise greatly improved are the peel strengthsobtained after 24 hours (adhesion).

IV Stability Testing

In order to test the stability to hydrolysis, 70% by weight solutions inIB of inventive and noninventive copolymers were prepared and werestirred under nitrogen in a closed vessel at 115° C. Samples were takenof these solutions after 36 and after 72 hours and the shear strengthsof the polymer films were determined as described in Section III above.The corresponding results are listed in Table 2.

TABLE 2 Shear strength as a measure of the hydrolysis resistance Polymerfrom Stirring times in Shear strength in Example hours minutes 1 0 12536 124 72 119 Comparative 1 0 109 36 12 72 1 2 0 122 36 120 72 118Comparative 2 0 107 36 29 72 18

As is evident from Table 2, the hotmelt pressure sensitive adhesives ofthe invention have a markedly higher hydrolysis resistance than apressure sensitive adhesive prepared using a noninventivephotoinitiator.

1. A copolymerizable photoinitiator comprising acetophenone orbenzophenone derivatives of the formula I,

wherein R is unsubstituted or substituted C₁ to C₃ alkyl, C₆ to C₁₀ arylor aralkyl having from 6 to 10 carbon atoms in the aryl moiety and from1 to 6 carbon atoms in the alkyl moiety.
 2. A copolymer comprising from0.01 to 10% by weight of acetophenone or benzophenone derivatives of theformula I in copolymerized form,

wherein R is unsubstituted or substituted C₁ to C₃ alkyl, C₆ to C₁₀ arylor aralkyl having from 6 to 10 carbon atoms in the aryl moiety and from1 to 6 carbon atoms in the alkyl moiety.
 3. A process for preparing acopolymer, which comprises polymerizing a mixture of ethylenicallyunsaturated compounds (monomers) comprising from 0.01 to 10% by weightof acetophenone or benzophenone derivatives of the formula I,

wherein R is unsubstituted or substituted C₁ to C₃ alkyl, C₆ to C₁₀ arylor aralkyl having from 6 to 10 carbon atoms in the aryl moiety and from1 to 6 carbon atoms in the alkyl moiety.
 4. The process as claimed inclaim 3, wherein said polymerizing step is initiated free-radically. 5.The process as claimed in claim 4, wherein said polymerizing step iscarried out in the form of a solution polymerization.
 6. A copolymerprepared by the process as claimed in claim
 3. 7. The copolymer asclaimed in claim 2, having a glass transition temperature of from −70 to+150° C.
 8. A UV-curable binder or adhesive comprising the copolymer asclaimed in claim
 2. 9. A pressure sensitive adhesive which comprises theUV-curable binder or adhesive as claimed in claim
 8. 10. The copolymeras claimed in claim 6 having a glass transition temperature of from −70to +150° C.
 11. A UV-curable binder or adhesive comprising the copolymeras claimed in claim
 6. 12. A pressure sensitive adhesive which comprisesthe UV-curable binder or adhesive as claimed in claim 11.